This invention relates to electrical tissue stimulators and more particularly, to tissue stimulator output circuits.
Electrical body tissue stimulators, such as nerve or muscle stimulators, cardiac pacemakers and the like are well known in the prior art. Electrical energy has been applied to body tissue using batteries, condensers, electrostatic charge generators and alternating current generators either for scientific curiosity or for treatment of various injuries and illnesses. Condenser discharge impulses of electrical energy into body tissue have been used therapeutically since the invention of the Leyden jar and their use continues today in implantable body stimulators, particularly cardiac pacemakers.
Typically, in the context of cardiac pacemakers, stimulation pulses have been delivered by means an electrode pair. Traditionally, electrode configurations for delivering cardiac pacing pulses have been referred to "unipolar" or "bipolar". In the unipolar configurations, one electrode is located adjacent heart tissue, and functions as a stimulation electrode. The second electrode is located remotely from heart tissue. In the bipolar configurations, both electrodes are typically located on or in the heart, with stimulation pulses applied between the two electrodes. In either case, the stimulation pulse typically comprises a single capacitive discharge applied between the two electrodes.
It should also be noted that in most modern pacemakers employing monophasic pulses a second, lower amplitude reverse polarity pulse is delivered through the pacing electrodes following the pacing pulse in order to quickly recharge the output capacitor. This second, lower amplitude pulse is not intended to be part of the stimulation pulse. Alternatively, multiphasic pacing pulses have also been suggested, typically also applied between a single pair of electrodes. For example, triphasic pacing pulses are disclosed in U.S. Pat. No. 4,343,956, issued to Cals et al.
Early implantable cardiac pacemakers, such as those shown in U.S. Pat. No. 3,057,356 and subsequent pacemakers up to the present date comprise small, completely implantable, transistorized and battery operated pulse generators connected to flexible leads bearing electrodes directly in contact with cardiac tissue. Demand cardiac pacemakers have traditionally employed a timing circuit, a stimulating circuit and a separate sensing circuit, all of which draw current from the power source. The stimulating circuits of such pulse generators have traditionally comprised constant current or constant voltage output circuits employing output capacitors which are charged to battery potential through a relatively high impedance and discharged through electrodes in contact with myocardial tissue to stimulate depolarization of the tissue. The output capacitors are typically recharged during the intervals between successive discharges. Examples of modern output circuitry for cardiac pacers are disclosed in commonly assigned U.S. Pat. No. 4,406,286, issued to Stein, and U.S. Pat. No. 4,476,868, issued to Thompson.
It has long been recognized in the pacing art that the stimulating energy delivered through traditional output circuitry is highly inefficient, and considerable efforts have been expended in reducing the energy required to pace the heart by development of smaller and more efficient electrodes, such as the small surface area, porous, steroid eluting pacing electrodes available from Medtronic, Inc. Nonetheless, pacing stimulation energies remain orders of magnitude higher than the stimulation energy found effective to commence the depolarization of a single myocardial cell.